The present invention relates to an intermediate transfer member used for an electrophotographic device or electrostatic recording device such as a copying machine or printer, wherein a toner image, which is formed by supplying a developer on the surface of an image forming member such as a latent image holder holding an electrostatic latent image, is once transferred and held on the surface of the intermediate transfer member and is then transferred on a recording medium such as a paper sheet; and an intermediate transfer device using the intermediate transfer member.
In an electrostatic recording process using a copying machine, printer or the like, there has been adopted a printing method including the steps of uniformly electrifying the surface of a photosensitive body (latent image holder), forming an electrostatic latent image on the photosensitive body by projecting light from an optical system on the photosensitive body to erase electrification of a portion where the light is irradiated, supplying a toner to the electrostatic latent image by a developing means such as a developing roller to form a toner image by electrostatic adhesion of the toner, transferring the toner image on a recording medium such as a paper sheet by a transferring means such as a transfer roller, and heating and fixing the transferred image on the recording medium by a fixing means such a fixing roller, to thus obtain a printed image.
In a color printer or a color copying machine, printing is basically performed in accordance with the above process; however, in the case of color printing, a color tone is reproduced using four kinds of toners corresponding to four colors (magenta, yellow, cyan and black), and consequently, there are required steps of obtaining a necessary color tone by superimposing these toners at a specific ratio. To achieve these steps, there have been proposed various methods.
As a first method, there is known a multiple developing method in which, to visualize an electrostatic latent image formed on a photosensitive body by supplying toners, development is performed by sequentially superimposing toners of four colors (magenta, yellow, cyan and black) to form a color toner image on the photosensitive body. This method allows the printing apparatus to be made relatively compact; however, it is disadvantageous in that control of color gradation is very difficult and thereby a high quality image cannot be obtained.
As a second method, there is known a tandem method using four photosensitive bodies aligned in a line. In this method, latent images formed on these photosensitive bodies are developed using toners of four colors (magenta, yellow, cyan and black) to form four toner images (magenta toner image, yellow toner image, cyan toner image, and black toner image), and the toner images are sequentially transferred on a recording medium such as a paper sheet in a superimposing manner, thereby reproducing a color image thereon. This method is advantageous in that a desirable image can be obtained; however, it is disadvantageous in that the printing apparatus has the four photosensitive bodies aligned in line, each being additionally provided with an electrifying mechanism and a developing mechanism and thereby it is enlarged in size and also increased in cost.
As a third method, there is known a transfer drum method using a transfer drum around which a recording medium such as a paper sheet is wound. Such a transfer drum revolves on its axis four times, and toner images of four colors (magenta, yellow, cyan, and black) formed on photosensitive bodies are sequentially transferred on the recording medium for each revolution of the transfer drum, to thereby reproduce a color image thereon. This method is advantageous in that a relatively high quality image can be obtained; however, it is disadvantageous in that there is a difficulty in winding a thick medium such as a post card, that is, there is a limitation to the kind of the recording medium.
In addition to the above multiple developing method, tandem method, and transfer drum method, there has been proposed an intermediate transfer method for ensuring a high quality image without enlargement of the size of the apparatus and also without limitation to the kind of the recording medium.
The intermediate transfer method adopts an intermediate transfer member formed into a drum or belt-like shape on which a toner image on a photosensitive body is transferred and held thereon once. To be more specific, the intermediate transfer method is carried out by forming toner images of four colors (magenta, yellow, cyan, and black) on four photosensitive bodies, sequentially transferring the four toner images from the four photosensitive bodies onto the surface of the intermediate transfer member to form a color image on the intermediate transfer member, and transferring the color image on a recording medium such as a paper sheet. In this method, since color gradation is adjusted by superimposing toner images of four colors, a high quality image can be obtained. Also, since the photosensitive bodies are not required to be aligned in a line like the tandem method, the size of the apparatus is not enlarged. Further, since a recording medium is not required to be wound around a drum, there is no limitation to the kind of the recording medium.
Such an image forming apparatus for forming a color image by the intermediate transfer method is shown in FIGS. 1 and 2, wherein FIG. 1 shows a type using a drum-like intermediate transfer member, and FIG. 2 shows a type using a belt-like intermediate transfer member.
Referring to FIGS. 1 and 2, reference numeral 1 indicates a drum-like photosensitive body which revolves in the direction shown by an arrow. The photosensitive body 1 is electrified by a primary electrifier 2, and is subjected to image exposure 3 for erasing electrification of an exposed portion. Thus, an electrostatic latent image corresponding to a first color component is formed on the photosensitive body 1. The electrostatic latent image is then developed with a magenta toner M as a first color toner using a developer 41 to form a magenta toner image as a first color image on the photosensitive body 1. The toner image is transferred on an intermediate transfer drum 20a (FIG. 1) or an intermediate transfer belt 20b (FIG. 2) (hereinafter, referred to as "an intermediate transfer member 20a or 20b") rotating in a state being in contact with the photosensitive body 1. In this case, the transfer of the image from the photosensitive body 1 to the intermediate transfer member 20a or 20b is performed by applying a primary transfer bias from a power supply 61 to the intermediate transfer member 20a or 20b at a nip portion between the photosensitive body 1 and the intermediate transfer member 20a or 20b. After the magenta toner image as the first color image is transferred on the intermediate transfer member 20a or 20b, the surface of the photosensitive body 1 is cleaned using a cleaning device 14. The first development/transfer operation by the initial rotation of the photosensitive body 1 is thus completed. Thereafter, the photosensitive body revolves on its axis three times, and a cyan toner image as a second color image, a yellow toner image as a third color image, and a black toner image as a fourth color image are sequentially formed on the photosensitive body 1 using developers 42, 43 and 44 for each revolution of the photosensitive body 1. The four toner images are sequentially transferred on the intermediate transfer member 20a or 20b in a superimposing manner for each revolution, to form a synthetic color toner image corresponding to the target color image on the intermediate transfer member 20a or 20b. It is to be noted that in the apparatus shown in FIG. 1, the developers 41 to 44 are sequentially exchanged for each revolution of the photosensitive body 1 to sequentially perform development by the magenta toner M, cyan toner C, yellow toner Y, and black toner B.
Next, a transfer roller 25 is abutted on the intermediate transfer member 20a or 20b on which the above synthetic color toner image is formed, and a recording medium 24 such as a paper sheet is fed from a paper cassette 9 into a nip portion therebetween. At the same time, a second transfer bias is applied from a power supply 29 to the transfer roller 25 so that the synthetic color image is transferred from the intermediate transfer member 20a or 20b to the recording medium 24 and is thermally fixed, by a fixing device 15, thereon as the final image. After the synthetic color image is transferred to the recording medium 24, the toner remaining on the surface of the intermediate transfer member 20a or 20b is removed by the cleaning device 35, and thereby the intermediate transfer member 20a or 20b is returned to the initial state to ready for the next image formation.
Such an intermediate transfer member 20a or 20b used for the intermediate transfer type image forming apparatus, which is in direct-contact with the photosensitive body 1, toner image, recording medium 24 and the like, is made from a soft elastic material. Further, in the case of the intermediate transfer member 20 a formed into the drum-like shape shown in FIG. 1, it is known that a conductive rubber-like elastic body 201 is laminated on a metal made drum base 200; and in the case of the intermediate transfer member 20b formed into the belt-like shape shown in FIG. 2, it has been known that a conductive rubber-like elastic body 201 reinforced by a canvass or a spirally wound thread (not shown) is formed into a belt-shape as shown in FIG. 4. As shown in FIGS. 3 and 4, there has been proposed a technique in which a surface layer 202 made from a resin is formed on a surface of the rubber-like elastic body 201 for preventing contamination of the photosensitive body, preventing adhesion of toner, and reducing a friction coefficient.
Incidentally, in the electrophotographic process or electrostatic recording process using such an image forming apparatus, ozone at a high concentration is produced in the electrifying and transfer steps, and accordingly, each of members constituting the image forming apparatus requires a high durability to ozone.
In the case of the above-described intermediate transfer member, however, it has been not apparent how the intermediate transfer member should be tested to judge whether or not the intermediate transfer member has a sufficient durability to ozone or it has been not apparent what kind of material should be used for the intermediate transfer member to ensure a sufficient durability to ozone.
Thus, a measure to shield members from ozone by covering the members with a metal material has been made; however, such a measure leads to the increased cost, and further a power supplying condition of the image forming apparatus has been restricted to suppress occurrence of ozone, causing a problem in design of the apparatus.
The intermediate transfer member might be also exposed to a high temperature upon fixing of an image, resulting in the thermal deterioration together with the above-described deterioration due to ozone. In particular, a rubber material used for an elastic layer is severely deteriorated due to these heat and ozone effects, and to cope with such an inconvenience, the rubber material has been added with an ozone-deterioration preventive agent and a thermal aging preventive agent.
The addition of an ozone-deterioration preventive agent and a thermal aging preventive agent in a rubber material, however, may contaminate a photosensitive body, toner, a paper sheet and the like, with a result that image formation becomes defective. To prevent such contamination, the above surface layer made from a resin has been formed on an elastic layer made from a rubber material. In this case, however, to ensure a sufficient contamination preventive effect, a surface layer having a relatively large thickness of several tens to several hundreds .mu.m must be formed, and the formation of such a thick surface layer makes higher a surface hardness of the intermediate transfer member. This presents problems in causing wear of a photosensitive body in contact with the member and fusion of toner.
The intermediate transfer member also requires a resistivity in a medium resistivity range of 10.sup.8 to 10.sup.14 .OMEGA.cm for applying a suitable vias voltage to an objected to be applied with a voltage such as toner or a paper sheet without occurrence of leakage and also breakage of a member due to the voltage. In this case, for an intermediate transfer member having a surface layer, the resistivity of the member becomes generally higher by the presence of the surface layer, and consequently, an elastic layer of the member requires a volume resistivity lower than that of an elastic layer of a member having no surface layer, more specifically, a volume resistivity in a semi-conductive range of 10.sup.4 to 10.sup.9 .OMEGA.cm.
However, it is very difficult to control a volume resistivity of a rubber material for an elastic layer in the above semi-conductive range. That is, it is extremely difficult to obtain a specific volume resistivity by addition a general conductive carbon material in a usual rubber material, so that there occur large variations in resistivity among and in each of elastic layers of intermediate transfer members obtained. This makes it difficult to stably obtain a member having an elastic layer having a uniform resistivity. On the basis of a graph (called percolation curve) obtained for rubber compositions whose volume resistivities are adjusted by addition of carbon, in which the abscissa indicates added amounts of carbon and the ordinate indicates volume resistivities of the rubber compositions, the volume resistivity is rapidly dropped at a specific added amount of carbon, and then kept constant. From this percolation curve, it becomes apparent that the variation in volume resistivity of a rubber composition is due to the rapid change in the percolation curve.
Accordingly, it is expected to develop an intermediate transfer member which is excellent in resistance to ozone and heat resistance,, and which is capable of stably exhibiting a preferable performance for a long period, of relatively easily adjusting a resistivity, and of reducing a variation in resistivity.
On the other hand, in the case where the surface layer 202 formed on the surface of the above intermediate transfer member is made from a resin material, the surface layer 202 may be peeled from the rubber-like elastic layer 201 or cracked because a resin material is generally harder than a rubber material and is difficult to follow elastic deformation of the rubber material. In this case, it may be considered to use a soft resin material; however, the soft resin material cannot be actually used for the surface layer 202 of the intermediate transfer member because the soft resin material is large in plastic deformation and also large in viscosity and friction coefficient. Besides, a urethane resin having a specific composition can preferably follow elastic deformation of a rubber material, and it has been proposed to use such a urethane resin for the surface layer 202; however, this urethane resin has a problem that the resistivity is large varied depending on an environment, to thereby make large the dependence of environment on the resistivity of the intermediate transfer member.
An intermediate transfer member has been also proposed in which the entire resistivity thereof is adjusted in a preferable medium resistivity range of 10.sup.3 to 10.sup.14 .OMEGA.cm by a method wherein the rubber-like elastic layer 201 is made from a rubber material whose volume resistivity is set at a relatively controllable value in a range of 10.sup.3 to 10.sup.9 .OMEGA.cm and the surface layer 202 is made from a resin having a relatively high volume resistivity. However, in the case where the resistivity of the intermediate transfer member is adjusted by controlling the volume resistivity of the surface layer 202, there occurs a problem. That is, since a general resin material has a high volume resistivity, such a high resistivity is required to be lowered typically by addition of a conductive material such as carbon. This requires complicated steps, for example, a step of preparing a paint in which a conductive material is uniformly dispersed, a step of forming a coating film in which the conductive material is uniformly dispersed, and the like, to degrade the production efficiency and increase the production cost.
Accordingly, it is expected to develop a surface layer which is capable of preferably following elastic deformation of an elastic layer, of easily adjusting a resistivity of the member, and of satisfying functions of preventing contamination of a photosensitive body or the like, preventing adhesion of toner, and reducing a friction coefficient.
Further, for an intermediate transfer member for transferring and holding a toner image formed on an image forming body such as a photosensitive drum onto its surface once and transferring the toner image onto a recording medium such a paper sheet, both in a step of holding a toner image transferred from the image forming body onto the surface of the intermediate transfer member and in a step of transferring the toner image from the intermediate transfer member onto the recording medium, a toner retention force for holding toner on the surface of the intermediate transfer member is an important factor exerting a large effect on a transfer performance.
The toner retention force on the surface of an intermediate transfer member, however, has been judged by testing the intermediate transfer member mounted on a copying machine or a printer, although it is an important factor exerting a large effect on the transfer performance, and the intermediate transfer member in terms of the toner retention force has been suitably designed by repeating try-and-error on the basis of the test results. And, at present, it is not apparent what kind of material should be selected and how the material should be used to obtain a desired intermediate transfer member.
Accordingly, it is expected to develop an intermediate transfer member used for an electrophotographic system or an electrostatic recording system such as a copying machine or a printer, which is capable of ensuring a preferable toner retention force by certainly evaluating the toner retention force without the need of carrying out any actual test.